This invention relates to an input integrated circuit and in particular an input integrated circuit to which different kinds of input signals are selectively supplied.
In an electronic circuit technique it is customary to properly select different levels of input signals to produce different states of outputs. In an input circuit having two power sources one of which has a high level and the other of which has a low level, for example, an input terminal externally supplied with signals is connected through a resistor to either one of the power sources. Now suppose that the input terminal is connected to the low level power source and in the "pull down" state. Where it is desired to selectively generate a "1" level (for example, a positive voltage level) and a "float" level signal from the input terminal it is necessary to provide a switch for connecting the input terminal to the positive voltage level. Suppose that the input terminal is connected through the resistor to the high level power source. In this case it is required that a switch for connecting the input terminal to the negative voltage source be provided to selectively generate a "0" level (for example, a negative voltage level) and the "float" level signal. Further suppose that the input terminal is selectively connected through a changeover switch to the positive voltage source or the negative voltage source. In this case it is possible to generate a "1" level or "0" level signal from the input terminal.
According to the above-mentioned three methods two states of signals can be selectively inputted to one input terminal. In this case, however, only one input level acts actively upon, for example, a logical circuit connected to the input terminal.
Recent rapid advance of an IC technique permits a multi-function circuit to be formed on a single chip. Such a multi-function trend of the integrated circuit leads to an increase in the number of input signals and an addition of a non-integrable portion of the circuit to the chip by means of an external connection element. If external terminals of the integrated circuit are increased in number with an increase in the number of input signals a corresponding chip size becomes bulky, incurring a high cost at the "packaging" step. Therefore, a great effort must be exercised as to how much the input and output terminals or the external connection elements can be reduced in number. In the above-mentioned method one input terminal is required to generate two states of signals. In the case of the two power sources two input terminals are required to obtain three signal states which are switched through a switch. One example is an electronic watch in which three signal states are obtained by effecting a switching between the two input terminals. Generally, the electronic watch requires a supply of a plurality of input signals so as to effect a mode switching and a time correction. In this case, the transfer of input signals is effected by supplying signal levels to two input terminals by the operation of at least two switches. If a plurality of switches are so employed, inconveniences are encoutered from the point of a size and cost. There is a demand for an integrated circuit capable of a similar control by one input terminal and one switch.